A permanent magnet motor (permanent magnet synchronous motor) is easy to perform maintenance on because it does not have a mechanical rectifying mechanism, and also has high efficiency because it utilizes a permanent magnet. Therefore, a permanent magnet motor is widely used as a motor for an electric automobile, an industrial machine, a compressor of an air conditioner and the like.
FIG. 8 is a cross-sectional view of an internal configuration of a typical permanent magnet motor. The motor M10 shown in FIG. 8 is a permanent magnet synchronous motor having a two-pole and three-slot structure including a rotor 1 (which may be of either the internally embedded magnet type or the surface magnet type) and a stator 2 of the concentrated winding structure. The rotor M10 is configured to be driven by phase alternate current applied to each stator winding 4 wound around the stator iron core 3.
The controller for a permanent magnet motor using the dq coordinate conversion performs control as follows. First, the phase current flowing through the stator winding 4 of the motor M10 is detected, and then the phase current of the rest frame coordinate system is converted into a d-axis current value and a q-axis current value of the rotating frame coordinate system. Then, the proportional-integral control is performed so that the d-axis current value becomes a d-axis current command value, thereby generating a d-axis voltage value. Also, the proportional-integral control is performed so that the q-axis current value becomes a q-axis current command value, thereby generating a q-axis voltage value. Then, these d-axis and q-axis voltage command values are converted into phase voltage command values of the rest frame. The voltage indicated by the phase voltage command values is applied to the stator winding 4. Then, the phase current of sinusoidal shape indicated by the d-axis and q-axis current command values is applied to the stator winding 4, thereby causing the predetermined output torque to occur.
In the permanent magnet motor of this kind, eccentricity is caused in a rotating rotor by the magnetic attractive force acting on the rotor. FIGS. 9A to 9F are schematic diagrams of eccentricity of the rotor 1 according to the typical permanent magnet motor M10. In this example, the rotor 1 rotates 180 degrees in the counterclockwise direction as shown in FIG. 9A through FIG. 9F (see the bold line marked on a part of the surface of the rotor 1 in the drawings). At the same time, the center of the rotor 1 is rotated 360 degrees in the clockwise direction by the magnetic attractive force (see the arrow in the radial direction in the drawings). Accordingly, in a permanent magnet motor having the two-pole and three-slot structure, eccentricity occurs with frequency of twice the mechanical rotation frequency of the rotor 1, causing the rotary bending vibration.
As a usage of a permanent magnet motor, an electric supercharger or a generator can be considered for instance. However, such usages may cause the rotation frequency of the rotor 1 to reach more than tens of thousands rotations inclusive, increasing the frequency of rotary bending vibration as well. This may lead to the fatigue fracture of the rotor 1. Especially, since the rotor in these usages is rotatably supported by a sliding bearing, which has a large backlash compared to the roller bearing, eccentricity is likely to occur in the rotor 1 as well as the fatigue fracture due to the rotary bending vibration.
As described above, it is a major problem for the permanent magnet motor to take a countermeasure for the rotary bending vibration of the rotor. Patent Document 1 is an example of the countermeasure technology of this kind. In Patent Document 1, the motor is stopped when rotary bending vibration is detected in the rotor so as to prevent the rotor from being fractured by the rotary bending vibration.